This invention generally relates to an apparatus and method for providing improved heat transfer in systems wherein heat transfer surfaces are present in a body of slurry. More particularly, this invention is directed to an improved crystallization apparatus and method wherein gas bubbles are used to provide improved heat transfer in crystallization systems wherein heat exchange surfaces are submerged in a slurry body. In this regard, an important aspect of this invention is directed to a new and improved surface-cooled fluid bed crystallizer which includes a gas distributor that discharges gas bubbles (typically air) upwardly through the magma onto and around heat exchanger tubes or plates contained therein to provide improved heat transfer between the cooling liquid and magma and economical production of a crystal product of desired particle size.
Surface-cooled crystallization systems utilizing heat exchanger surfaces (e.g. heat exchanger tubes or plates) submerged in a slurry contained in a vessel have been commonly utilized in applications where the temperature level is low making vacuum crystallization impractical or where the solution boiling point elevation is high. Typically, these systems operate at atmospheric pressure and are characterized by relatively poor heat transfer rates brought about, in part, by the build-up of crystals on the heat exchanger surfaces and relatively low fluid velocity at the heat exchanger surfaces.
Efforts to overcome some of the disadvantages of the aforementioned surface-cooled crystallizers have involved the use of mechanical agitation devices. For example, in an agitated batch crystallizer, water is circulated through the cooling coils and a solution is agitated by propellers on a central shaft. This agitation increases the rate of heat transfer and tends to keep the temperature of the solution more uniform. Additionally, it serves to keep the fine crystals in suspension, giving them an opportunity to grow uniformly instead of forming large crystals or aggregates. Shock imposed by the mechanical agitation device, however, introduces unwanted nucleation resulting in the formation of product crystals which are smaller in size. Other disadvantages of the agitated batch crystallizers are that the system is a batch or discontinuous method and also that the solubility is lowest in the stagnant film on the surface of the cooling coils. Consequently, crystal growth is most rapid at this point and the coils rapidly build-up a mass of crystals which serve to decrease the rate of heat transfer.
The use of air agitation in prior art crystallization processes has generally involved the introduction of such air in locations in the slurry body wherein no heat exchange surfaces are present. For example, U.S. Pat. No. 3,599,701 describes a system wherein air is injected into a slurry in a crystallization zone for the purpose of concentrating the slurry and creating sufficient turbulence in the crystallization zone for preventing or minimizing classification so that all areas in the crystallization zone have a substantially uniform composition. Similarly, U.S. Pat. No. 3,883,311 describes a reaction crystallizer which utilizes an air distributor located below a draft tube contained within a body of slurry. Neither of these references utilizes air bubbles to bathe any heat exchanger surfaces.
In accordance with an important aspect of the present invention, the problems and disadvantages of these prior art crystallizer systems are overcome by providing a surface-cooled crystallization apparatus and method wherein submerged heat exchange surfaces are bathed with a stream of gas bubbles. In particular, the heat exchange surfaces (heat exchange tubes or plates) through which a coolant, such as cold water or brine is passed, are submerged in a body of slurry. The coolant establishes or maintains the magma at crystallization facilitating temperatures and a gas distributor located below the heat exchange surfaces produces a supply of bubbles of a gas, preferably air, which rise upwardly through the slurry body onto and around the heat exchange surfaces. This gas increases the localized velocity at the heat exchange surfaces, improves heat transfer, reduces crystallization on the heat exchange walls and gently keeps the crystals in suspension thereby avoiding unwanted nucleation as is characterized by the use of mechanical circulation devices. This use of the gas bubbles enables the conversion of a quiescent crystallizer into a crystallizer with agitation without the drawback associated with the use of mechanical agitation. Further, by continuously removing the depleted magma and crystals, the crystallizer can be operated in a continuous mode.
The present invention also can be advantageously used in other forms of crystallization systems including, for example, systems wherein, instead of air, a reactant gas is utilized to bathe submerged heat exchange tubes or plates through which a cooling or heating medium is passed depending upon whether the particular application requires either the input or removal of heat from the slurry body in order to maintain crystallization conditions therein. As such, the previously noted benefits of this invention are likewise achievable in reactive type crystallizers having heat exchange tubes or plates submerged in a slurry body with a gas distributor located below such heat exchange tubes or plates. The reactant gas is discharged upwardly through the slurry so that it both produces the desired reaction in the slurry body and also bathes such heat exchange surfaces as previously described.
It is, therefore, an important object of the present invention to provide an improved crystallization apparatus and method.
Another object of the present invention is to provide an apparatus and method characterized by improved heat transfer in systems wherein heat exchange surfaces are located within a slurry body and wherein mechanical agitation or forced circulation devices are not required.
Another object of the present invention is to provide an improved crystallization apparatus and method wherein gas bubbles are discharged onto and around heat exchange surfaces located within the slurry body to both agitate the slurry body and to prevent or minimize the formation of crystals on the heat exchange surfaces.
Another object of the present invention is to provide an improved surface-cooled fluid bed crystallizer and method wherein heat exchange tubes or plates through which a coolant is passed are submerged in a magma body in overlying relationship to a gas distributor that discharges gas bubbles upwardly through the slurry body onto and around the heat exchange tubes or plates to provide both improved heat transfer between the cooling liquid and the magma, minimize the formation of crystal particles on the heat exchange tubes or plates, and to impart gentle agitation to the slurry body without incurring unwanted nucleation.
Another object of the present invention is to provide an improved reaction type crystallizer wherein a cooling or heating medium is circulated through heat exchange tubes or plates which are submerged in a slurry body in overlying relation to a gas distributor that is supplied with a reactant gas that produces the desired reaction in the slurry and both agitates the slurry (without requiring mechanical agitation) and prevents or minimizes the formation of crystals on the surfaces of heat exchange tubes or plates.